Fluoropolymer based coatings have been used extensively as protective and decorative coatings because of their known stability and durability under adverse or prolonged weathering and chemical exposures.
Solvent based fluoropolymer coatings are well known in the art. They are prepared by dispersing the fluoropolymer in a suitable solvent, applying the dispersion onto a substrate and subjecting the coated substrate to heat. The solvent used is generally an organic solvent which does not interact with the fluoropolymer at ambient temperatures, but exhibits solvent activity at higher temperatures. Organic solvents are health hazards and environmentally damaging compounds and the recovery of these solvents is a costly and time-consuming procedure. Therefore, a need has arisen in the industry for cost-effective processes for preparing fluoropolymer based powders which produce coating films with flow and levelling characteristics comparable to those exhibited by solvent based dispersions.
Solventless processes for preparing pigmented poly(vinylidene fluoride) based coatings are known in the art. For example, British Patent Specification No. 2,194,539 A, U.S. Pat. Nos. 4,770,939 and 5,030,394 disclose a mixture of poly(vinylidene fluoride) resin and a thermoplastic acrylic resin, such as poly(methylmethacrylate), which is extruded and pelletized. To obtain proper dispersion of the ingredients, the dry mixture must be extruded at about 220.degree. C. and the resultant hot extrudate must be rapidly chilled or quenched with cooling water before it may be pelletized. This rapid cooling process results in particulates which are highly amorphous in form. After pelletization, the resulting pellets are pre-cooled to a temperature of about -150.degree. C. and ground cryogenically in a hammer mill. Cryogenic grinding is defined in the specification of both patents as grinding at a temperature no higher than -50.degree. C. The extremely low grinding temperatures are accomplished by the introduction of substantial amounts of liquid nitrogen. Particles which are larger than a predetermined size are then sieved out.
The principal disadvantage of the process disclosed in British Patent Specification No. 2,194,539 A, U.S. Pat. Nos. 4,770,939 and 5,030,394 is that a large amount of liquid nitrogen is needed to cool and embrittle the amorphous pellets prior to grinding and to simultaneously maintain the hammer mill at a grinding temperature of less than -50.degree. C. Such cryogenic grinding procedure using a large amount of liquid nitrogen is expensive and limited in its application in that only specially designed grinding mills which can withstand continuous operation under the extremely low cryogenic temperatures can be used. The particles that result from this process is relatively coarse in size distribution and produce coating films with inferior flow and levelling characteristics.
The compositions and the process for preparing powder coatings as described in this invention yield fluoropolymer based powder coatings with unusually good flow and levelling characteristics with films comparable to those obtained from solvent-based fluoropolymer coatings.